The invention relates to a switching unit which comprises a non-linear asymmetric switching element of a thin layer of semiconductor material and having a lower contact and an upper contact.
The invention also relates to a display device in which such a switching unit is used.
As is known, such a display device is suitable, for example, for displaying datagraphic information and video information by means of an array of pixels including a passive electro-optical display media such as liquid crystals, electrophoretic suspensions or electrochromic materials the pixels electrically connected to row electrodes and to column electrodes via switches.
The use of such switching elements in such display devices provides the possibility of increasing the number of rows of pixels to be driven. In fact, the switching elements provides a sufficiently high threshold so that a pixel cannot be driven if the relevant row is not selected. In addition the switching element ensures that the information presented while a row electrode is being driven subsists across a pixel during the period when the other row electrodes are driven. The switching element also prevents a pixel from undergoing an effective voltage during the period when it is not driven.
European Patent Application No. 0296662, to Kuijk, describes a display device in which the pixels are given voltages by means of switching units which comprise an asymmetric non-linear switching element, for example, a diode. The voltage across the pixels is accurately adjusted by means of these units after the associated capacitances are first discharged or charged, if necessary, beyond the range of transition in the transmission/voltage characteristic of the electro-optical medium. To this end, the switching unit also comprises a non-linear switching element, for example, a MIM (metal-isolator-metal) element which is arranged (anti) parallel to the diode.
Such diodes are usually made of amorphous silicon. One of the drawbacks of amorphous silicon is its sensitivity to light, resulting in the off-current I.sub.off of such diodes being unacceptably high when they are irradiated with light. The light sensitivity problem may also occur (though to a lesser extent) when using other semiconductor materials, for example, polycrystalline silicon.
Moreover, the switching unit comprises two separate elements which reduces the effective size of the associated pixel and sometimes requires different fabrication technologies.